(1) Field of the Invention
The present invention relates to an adsorbent for iodine and organic iodine compounds (hereinafter referred to as "iodine compounds" collectively). More particularly, it relates to an adsorbent for removing radioactive iodine compounds contained in exhaust gases.
(2) Description of the Related Art
To prevent environmental pollution, the discharge of radioactive substances into outer air is strictly controlled and, to this end, an emergency filter for removing radioactive iodine compounds is provided in a reactor building at a nuclear power plant to prevent the emission of radioactive iodine compounds at the time of an accident. Such a radioactive iodine compound-removing filter is also attached to the ventilation system of a reactor building. It is also required that a radioactive iodine compound-removing filter be provided when reprocessing spent nuclear fuel, or in a laboratory where a radioactive substance is used. Thus, the nature of an adsorbent for iodine compounds used for these purposes has become very important.
As the adsorbent for iodine compounds, there have been used iodine compound-impregnated active carbon and amine-impregnated active carbon.
The above-mentioned active carbon adsorbent is not costly but is defective in that the iodine compound-removing efficiency is reduced at a high temperature or under high humidity conditions. As an adsorbent having a removing capacity which is not reduced even under high humidity conditions, an adsorbent comprising a silver loaded X-type zeolite or mordenite has been developed [see U.S. Pat. No. 3,658,467 and Ceramics Japan, 16 (11), 927-928 (1981)].
However, this silver-zeolite adsorbent is very costly because silver is expensive and is contained in an amount as large as 20 to 40% by weight. Furthermore, the zeolite is extremely hydrophilic, and therefore, if the humidity is very high, the removing efficiency thereof becomes unsatisfactory under high humidity conditions.
Under this background, the development of a novel adsorbent containing a reduced amount of silver and exhibiting a high iodine compound-removing efficiency, even under high humidity conditions, is strongly desired.